1. Field of the Invention
The present invention relates generally to manufacturing processes for semiconductor integrated circuit devices. More particularly, to an improved method of applying a lower viscosity coating liquid, such as a photoresist solution and an anti-reflection film coating (ARC) solution, onto a semiconductor wafer to form film with uniform surface.
2. Description of the Related Art
Recently, wafer size tends to increase as device circuits are miniaturized. With this tendency, a low-viscosity photoresist solution and low-viscosity anti-reflection coating film solution (hereinafter referred to as “ARC solution”) have been increasingly used. For forming a thicker film coating the low-viscosity solution onto the wafer, the common method reduces the rotation speed of the wafer to increase coating thickness. However, the thickness of the coating film is thicker than the predetermined thickness and the uniformity is worse.
In general, the traditional coating process substantially comprises five steps.                Step I: spraying a solvent (also called prewetting)        Step II: drying the solvent        Step III: dispensing a coating solution        Step IV: spreading the coating solution        Step V: cleaning the wafer        
An example using the traditional coating method to form a photoresist film with predetermined thickness of 1350 Å is given in Table 1 and FIG. 1 which shows the wafer rotation speeds in each step.
TABLE 1RotationAcceleration/Timespeeddeceleration ratestep(sec)(rpm)(rpm/sec)1.00I1.50Spraying a solventII3.0200010000III2.6100010000Dispensing a photoresistsolution (2.5 cc/2.2 sec)IV95.052510000Spreading the photoresistsolutionV1.0200010000Cleaning edge of the wafer5.02000Cleaning edge/backsideof the wafer1.0250010000Cleaning edge of the wafer5.0300010000Cleaning edge of the wafer1.0010000
Referring to Table 1 and FIG. 1, after the wafer is positioned in a spin coater and the rotation speed is kept at 0 rpm/sec, solvent is sprayed on the surface of the wafer for 1.5 seconds to prewet the surface of the wafer.
The solvent is dried while the wafer is accelerated from 0 rpm to 2,000 rpm and kept at 2,000 rpm at a rate of 10,000 rpm/sec for 3 seconds.
The photoresist solution is dispensed on the surface of the wafer at a rate of 2.5 cc/2.2 sec for 2.6 seconds while the wafer is decelerated from 2,000 rpm to 1,000 rpm at a rate of 10,000 rpm/Sec.
The coating solution dispensed on the wafer is spread for 95 seconds while the wafer is decelerated from 1,000 rpm to 525 rpm at a rate of 10,000 rpm/sec to form the photoresist film with predetermined thickness.
The wafer is accelerated from 1,000 rpm to 2,000 rpm at a rate of 10,000 rpm/sec and kept at 2,000 rpm total for 1 second while the edge of the wafer is cleaned. The wafer is kept at 2,000 rpm for 5 seconds and the edge and backside of the wafer are cleaned. The wafer is accelerated from 2,000 rpm to 2,500 rpm at a rate of 10,000 rpm/sec while the edge of the wafer is cleaned for 1 second. The wafer is accelerated from 2,500 rpm to 3,000 rpm at a rate of 10,000 rpm/sec while the edge of the wafer is cleaned for 5 seconds.
However, while the traditional coating method is used to form the photoresist film in 8 inch wafers, the obtained photoresist film has poor uniformity. As shown in FIG. 2, the average thickness of the photoresist layer is 1331.19 Å, and the thickness variation is as high as 31.07 Å. In this manner, some problems may occur in subsequent processes such as photolithography and etching. For example, the width of the conducting line formed after etching is not uniform, and yield is decreased. These problems will become more serious with increases in the size of the wafer.